Why does Louis don’t know about Gabriella? I understand why Lestat didn’t tell him, but Armand. Why does Armand not mention Gabriella? In the 77 years he was with Louis, why doesn’t he mention her? Why didn’t he say ‘hey remember the guy you are trying to spite by being with me? did he ever tell you about his mother? NO? O she is not dead I met her , yeah horrible person and btw they were fucking’?
I think it's implied that Lestat was the elephant in the room between them.
In Paris, Louis avoided talking about Lestat because he knew he and Claudia could be punished for "killing" him. He tried not to even think about Lestat. (Though obviously Dreamstat shows he was not successful at that).
Then in San Francisco, Armand says that Lestat's name was "un-uttered in our home for 23 years". They don't talk about him. They pretend to have moved on from him. So Armand is furious to hear Louis mention Lestat's name so many times on the tapes, and he knows Louis still loves Lestat, even if Louis won't admit it to himself.
After that, Armand had altered Louis's memories of that night in San Francisco, so it would have been kind of awkward, after 23 three years, for him to suddenly bring up Lestat and say "by the way, he was sleeping with his mom.".
Besides, I think Armand knew that, if Louis believing that Lestat was responsible for Claudia's death didn't stop Louis from loving him, finding out that he was sleeping with his mother wasn't going to do it either.
